Selective protection systems are known which combine convolutional coding with digital modulation. More particularly, for enhancing the performance of such a system in terms of bit error rates, a new class of convolutional codes called turbo codes has been described in the document entitled: "Near Shannon limit error-correcting coding and decoding: Turbo-codes (1), C. BERROU, A. GLAVIEUX, P. THITIMAJSHIMA, Proceeding of ICC '93, Geneva, May 1993, pp. 1064-1071. These turbo codes have a performance, expressed in bit error rates, close to theoretical limits provided by Shannon. They are parallel concatenated convolutional codes. These convolutional codes are based on systematic punctured recursive codes, that is, they are derived from codes of rate 1/2 with which one input bit gives rise to two output bits, the puncturing being applied to the bits themselves. The concatenation as such is realized by an appropriate interleaving of the data that contain the information.
At the receiver end, the iterative decoding, called turbo decoding, consists of decoding the received symbols several times to improve the performance of the system in terms of bit error rates. The successive iterative decodings are carried out by a single soft-decision Viterbi decoder. This decoder differs from a conventional Viterbi decoder which makes a hard decision. In this document, the soft decisions on the output of the decoder produce a hard-decision reliability ratio, that is, a correct decision likelihood. The decoder is then followed by a deinterleaving of the output data.
However the use of turbo codes followed by an iterative decoding, as disclosed in said documents does not suppress the limitations of punctured systematic codes with codes of rate 1/2, especially when the latter are applied separately from the digital modulation with which they co-operate. More particularly, a disadvantage of these codes is that they are adapted only to modulations having a low spectral efficiency (less than or equal to two bits/s/Hz), such as the MDP2 and MDP4 phase modulations. A bit rate equal to a maximum of twice the band used corresponds to these codes. For augmenting the rate for a fixed occupied band, one could look for utilizing high spectral efficiency modulations of the quadrature amplitude modulation type (QAM). But the use of the punctured convolutional codes described above, juxtaposed with QAM modulations, does not provide optimum performance, because these codes have been designed irrespective of the modulation.